(110)TiO2

Article pubs.acs.org/JPCC

Periodic DFT+D Molecular Modeling of the Zn-MOF-5(100)/(110)TiO2 Interface: Electronic Structure, Chemical Bonding, Adhesion, and Strain Filip Zasada,* Witold Piskorz, Joanna Gryboś, and Zbigniew Sojka Faculty of Chemistry, Jagiellonian University, ul. Ingardena 3, 30-060 Krakow, Poland S Supporting Information *

ABSTRACT: Electronic structure, bonding characteristics, adhesion, and stress energy of the Zn-MOF-5(100)/(110) rutile interface were modeled by using periodic DFT+D calculations, corroborated by simulation of high resolution transmission electron microscopy (HR-TEM) images. Adjustment of the flexible metal−organic framework (MOF) moiety to the rigid rutile substrate was achieved within a supercell comprised of (1 × 1) Zn-MOF-5 and (4 × 9) TiO2 units. It was shown that binding of the Zn-MOF-5 layer takes place via bidentate 1,4-benzenedicarboxylate (BDC)−titania bridges. A coherent interface can be formed with the minimal periodicity along the [11̅0] direction defined by nine Ti5c adsorption sites (9 × 2.96 Å = 26.64 Å) and two consecutive linkers of the Zn-MOF-5 chain (2 × 12.94 Å = 25.88 Å). The MOF part is tuned to the oxide substrate by tilting the BDC linkers by 10° and twisting around their long axis by 34°. The resultant lattice strain of the Zn-MOF-5 layer was equal to ε[001] = 0.31% and ε[110̅ ] = 2.86%, and the associated stress energy to σtotal = 4.8 eV. Pronounced adhesion energy of the Zn-MOF-5 layer deposited on the rutile surface (−0.33 eV/nm2) stems from the sizable dispersion (−0.39 eV/nm2) contribution, counterbalancing the unfavorable lattice strain and bonds distortion components. The calculated density of states structure of the Zn-MOF-5(100)/(110)TiO2 interface revealed that it can be described as an electronically coupled, staggered (Type II) charge injection system, where a photoinduced electron may be directly transferred from the Zn-MOF-5 moiety to the conduction band of the titania substrate.

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STEM, or electron diffraction.20−23 The first successful ultrahigh resolution imaging of MOF-5 crystals using aberration-corrected cryo-TEM (80 keV) has been published recently by Wiktor et al.,24 opening a new field for direct structure observations of those fragile materials with atomic resolution. For a number of important applications (e.g., functionalized membranes and sensors,25 solar materials, electronic and light emitting devices26−28), homogeneous, compact, dense, welloriented, and defect-free MOF thin films (surface-attached metal−organic framework layers known as SURMOFs) are of special interest.29−31 However, despite extensive studies in this field only a few reports on the deposition of MOF on solid substrates are available at present.30,32,33 Following Fischer,32 two classes of MOF films can be distinguished: polycrystalline films and thin surface MOF layers. Polycrystalline films may be regarded as randomly oriented MOF crystallites covering the surface of a substrate in a compact or scarce fashion. Their thickness may arrive at a micrometric scale, often mere confinement rather than substrate related effects are responsible for orienting the growth. However, chemical interactions with the surface may

INTRODUCTION Owing to their unique structure and the related properties such as designable topology, tunable metrics, and versatile functionalities, metal−organic frameworks (MOF) belong to the well-establish class of advanced hybrid materials of broad scientific interest.1−3 The MOF structure is based on a combination of two essential building components: an inorganic part (constituted by metal or metal−oxo clusters) and a functional organic part acting as a linker. Their ultrahigh apparent surface areas and inner porosities (up to 90% free volume),4 together with extraordinary chemical changeability of both the inorganic building units and the organic connectors, give rise to a wide range of spectacular applications. Indeed, they are used as gas storage5 and molecular sieving materials,6,7 catalysts for fine chemicals,8−10 drug delivery systems,11,12 or chemical sensors.13,14 Various synthetic strategies have been developed for designing and manufacturing a vast range of good-quality bulk MOF materials, and they have been reviewed several times elsewhere.15−17 Typically, MOF materials are prepared in the form of powders by conventional chemical methods, yet the resultant microcrystals are often not of the desired quality. Their size and shape are usually imaged by using scanning electron microscopy,18,19 and several interrelated transmission electron microscopy techniques such as high resolution transmission electron microscopy (HR-TEM), HAADF© XXXX American Chemical Society

Received: December 30, 2013 Revised: March 17, 2014

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dx.doi.org/10.1021/jp412756a | J. Phys. Chem. C XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry C

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METHODOLOGY For all calculations we used the DFT theory implemented in the Vienna Ab-initio Simulation Package (VASP).59,60 We employed the projector augmented plane wave (PAW) method61,62 for describing electron−ion interactions together with the PW91 exchange-correlation functional. The GGA functionals are well-known for giving reliable results in describing the electronic structure of titania and other akin oxide systems.63−65 The Zn-MOF-5 bulk model was built on the basis of an X-ray single-crystal diffraction study of Hailian et al.52 We used a unit cell of cubic symmetry (Fm3̅ m group) with the Zn32O104C192H96 stoichiometry, and the initial lattice constant of 25.86 Å.52 To model the rutile (110) surface, large enough to accommodate a (1 × 1) Zn-MOF-5 (100) surface cell, a (9 × 4) slab supercell with the stoichiometry of Ti216O432 was constructed by cleaving the optimized bulk crystal along the (110) plane. It consisted of nine TiO2 atomic layers (with a thickness of 9.01 Å) and a wide vacuum layer (42.0 Å) in the [110] direction, thick enough to prevent the slabs from unphysical mutual interactions. Taking into account the fact that the Zn-MOF-5 framework is much softer than that of the rutile one (vide infra), the initial size of the supercell was based on the lattice constant of the rigid TiO2 component. The lattice parameters of such a supercell were equal to a = 26.63 Å, b = 25.97, and c = 51.00 Å along the [11̅0], [001], and [110] directions of the rutile structure. Both the stoichiometry of the bulk TiO2 and the same structure of the top and bottom slab terminations were preserved. The geometry was optimized with the following constraints: five innermost atomic layers of TiO2 were fixed at the optimized bulk positions, while the remaining oxide layers were allowed to relax. Having geometry considerations in mind, a quantum chemical model of the Zn-MOF-5(100)/(110)TiO2 interface was constructed, assuming an attachment of the Zn-MOF-5 moiety via the organic subunits, and complete protonation of the terminal top and bottom 1,4-benzenedicarboxylate (BDC) linkers (BDC is a deprotonated form of TPA). Alternative junction via inorganic subunits was rejected for energetic reasons−see the Supporting Information, Figure S1. Upon binding to the Ti surface ions, the released protons of the bottom linkers are accommodated by the exposed O2c ions (see below), whereas the free top BDC linkers remain fully protonated. The size of the interface model was adequate to accommodate a Zn-MOF-5 fragment with four BDC anchors to TiO2. The vacuum region of ∼10 Å was placed in the interface normal direction between the terminating −COOH groups and bottom of the TiO2 slab, to separate periodically repeated images and to avoid mutual interactions. For the interface optimization the titania ions in the bottom part were fixed (see above), while all the Zn-MOF-5 atoms were allowed to relax. An exceptionally large size of the whole system limited the sampling of the irreducible Brillouin zone to the Γ point only, with the cutoff energy set to 380 eV. For solving the Kohn− Sham equations, the self-consistent field (SCF) convergence criterion was set to energy change not larger than 10−5 eV between two successive iterations. To improve SCF convergence the Methfessel−Paxton smearing66 with σ parameter set to 0.1 eV was applied. Geometry optimization was performed until the changes in the forces acting on the ions were smaller than 0.001 eV/Å per atom. A semiempirical

promote favorable attachment of the deposited MOF material via, e.g., hydrogen or even covalent bonds, giving rise to partially or fully oriented films. The resultant SURMOFs are constituted by smooth, well-oriented (at least in one dimension) nanometric multilayers. Such ultrathin films of a versatile, on purpose tailored structure, texture, and patterning are interesting not only for the fundamental level studies, but also from the viewpoint of promising practical applications. The MOF thin films are usually obtained via liquid-phase epitaxy (LPE) on self-assembled monolayers (SAM),34 or a Langmuir−Blodgett layer-by-layer deposition.35 A typical example is provided by thiolate-based self-assembled monolayers on gold as templates for the HKUST-1 system.36−38 The MOF thin films can, in principle, be produced on the surface of any solid particle.39 Among them thin films on substrates such as silica, alumina, and titania produced via seeded growth or deposition from mother solution are reported.40−43 However, a problem often encountered therein is the lack of proper adhesion between the seeds and the substrate. This difficulty may be circumvented by proper functionalization of the oxide surface prior deposition.44 Such reactive seeding, for instance, has been adopted successfully by Hu et al. for growing MIL-53 on an alumina substrate.45 The microwave-assisted mother solution method, in turn, has been used for deposition of ZIF-8 and ZIF-22 on titania.46,47 Rutile nanocrystals in their thermodynamically stable shape exhibit low index facets, among them the (110) plane is predominant.48 This termination is relatively easy to prepare, also in the case of single crystals.49,50 Recently, it has been reported that the rutile (110) surface can be functionalized by a compact well-ordered monolayer of the terephthalic acid (TPA) molecules,44,51 commonly used as linkers in MOF synthesis. It has also been argued that such functionalization may be used for seeding purposes and generation of the nascent TPA/TiO2 monolayer, allowing for subsequent epitaxial grow of a more robust MOF thin film in a layer-by-layer fashion. While there are several reports available in the current literature on the research in this area involving titania as a substrate,47,42 the MOF−oxide interface is still not well understood at the molecular level. The specific MOF material addressed in this paper is ZnMOF-5 of the fcc structure.52 We explored, in particular, the possibility of formation of a compact and adhesive Zn-MOF-5 ultrathin film (monolayer) on the rutile substrate by modeling the structure, energetics, and intimate chemical interactions within the Zn-MOF-5(100)/(110) TiO2 interface, using the quantum chemical approach. Noting recent interest in MOF sensibilization of titania for emerging applications in photocatalysis, photonics, and dye-sensitized solar cells (DSSC) in particular,53−55 we calculated the density of states (DOS) structure of the Zn-MOF-5/TiO2 assembly as well. Furthermore, for inspiring future experiments and substantiating the elaborated model, we simulated HR-TEM pictures of the ZnMOF-5 layer on the TiO2 surface, specifying preliminary conditions for optimal imaging. Although there are several papers dealing with DFT calculations of the MOF structure and storage of small molecules inside those materials,56−58 it is, to our best knowledge, the first attempt to comprehensively describe a MOF/oxide interface by using molecular modeling, DOS calculations, and ab initio HR-TEM image simulation. B



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dispersion term, parametrized by Grimme,67 and successfully applied for modeling the interactions between hydrocarbon admolecules and oxide substrates, was added to the quantummechanical energies and gradients (DFT+D). In accounting for the electron correlation effects and the Coulomb on-site repulsion, for fair reproduction of the band gap we employed the DFT+U approach68,69 with the Hubbard parameter set to U = 5.0 eV for Ti ions.70 The bulk and the Young modules were calculated by fitting the calculated E/V values to the Birch− Murnaghan equation of state,71 with the relative RMS error of 1, due to their prohibitive size is unfortunately not yet possible at reasonable computational cost. Using the values obtained for the smallest (1 × 1)(9 × 4) cell we can, however, estimate the relevant adhesion energy for larger models in the following way: Eadh = Ecell‑stress + Ebond‑dist + Eanchor (all with inclusion of dispersion interactions). For instance, in the particular case of the well-matched (1 × 4) (9 × 35) cell with virtually no strain, we can reasonably assume that Estress ≅ 0 nm2 eV, with other contributions being close to the DFT calculated values for the smaller (1 × 1)(9 × 4) cell. Thus, taking into account the difference in the linker surface density (0.57 and 0.60 per nm2 for (1 × 1)(4 × 9) and (1 × 4) (4 × 35) models, respectively), we can estimate the adhesion

Figure 3. The optimized geometry of the Zn-MOF-5(100)/(110) TiO2 interface presented in the (1 × 1)(9 × 4) unit cell used in the calculations (a). Magnified picture of the BDC linker showing tilting and unsymmetrical stretching of the Zn−OTPA bonds upon attachment to the titania substrate (b). Color coding as follows: Ti, light gray; O, red; C, dark gray; H, white; and Zn, blue polyhedra.

oxygen, Oc, remains essentially intact. In both [001] and [11̅0] directions the same changes were observed in the first and the upper layers of the deposited Zn-MOF-5 film (Table 1). A different situation was found for the [110] direction, in which the interface and the upper layers were modified in a dissimilar way upon interaction with the rutile (110) surface. Obviously, the most pronounced changes in the bond lengths were noted for the first (interface) layer of the deposited Zn-MOF-5, and they were confined to the dicarboxylic linkers mainly (Table 1). The elongation of the Ccarboxyl−Cphenyl bonds by 0.13 Å is related to a distinct and asymmetric stretching of the Zn−OTPA bonds (2.31 Å /1.95 Å), associated with the already mentioned tilting of the linkers (see Figure 3b). In the case of the upper MOF layers, only slight changes in the bond length parallel to the [110] direction were detected, showing that structural constraints imposed by the rutile substrate are effectively accommodated by the first layer. As shown in the bottom row of Table 1, the carboxyl moieties of the BCD linkers were slightly deformed (see the variation of the OTPA−Ccarb−OTPA′ angles). Interface Energetics. The Zn-MOF-5(100)/(110)TiO2 interface was further analyzed in terms of the adhesion energy,

Table 1. Geometric Parameters of the Bulk and the Titania-Attached Zn-MOF-5 Layers (Observed Changes Are Indicated in Parentheses) Zn-MOF-5 layer: (1 × 1)(9 × 4) cell bulk Zn-MOF-5 distances/Å Zn−Oc Zn−OTPA Ccarboxyl−Cphenyl OTPA−Ccarboxyl angles/deg OTPA−Ccarboxyl−OTPA′

[11̅0] direction

[001] direction

1.95 1.94 1.94 1.50 1.26

1.94 1.96 1.96 1.51 1.23

125.86

130.90

(−0.01) (+0.02) (+0.02) (+0.01) (−0.03)

1.96 1.98 1.98 1.60 1.25 129.02 E

(+0.01) (+0.04) (+0.04) (+0.10) (−0.01)

[110] interface layer 1.94 2.30 1.95 1.63 1.23 129.86

(−0.01) (+0.36) (+0.01) (+0.13) (−0.03)

[110] upper layers 1.93 1.94 1.94 1.51 1.24

(−0.02) (+0.00) (+0.00) (+0.01) (−0.02)

131.02



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Table 2. Energetic Contributions to Adhesion Energy, Eadh, of the Zn-MOF-5(100)/(110)TiO2 Interfacea Eadh

Ecell‑stress

Ebond‑dist

Eanch

energy/eV

per nm2

per unit cell

per nm2

per unit cell

per nm2

per unit cell

per nm2

per unit cell

Etotal EDFT Edisp

−0.32 0.07 −0.39

−2.21 0.48 −2.69

0.69 0.67 0.02

4.80 4.65 0.15

0.53 0.53 0.00

3.68 3.67 0.01

−1.55 −1.13 −0.41

−10.69 −7.84 −2.85

a

Ecell‑stress stands for stretching of the Zn-MOF-5 layer, Ebond‑dist denotes energetic cost for adjustment of the MOF atomic positions to the TiO2 substrate, and Eanch stands for the energetic effect of chemical anchoring of the entatic Zn-MOF-5 part on the TiO2 substrate. EDFT (electronic energy) and Edisp (dispersion energy) merge together to give the total energy (Etotal) for each effect (Eadh, Ecell‑stress, Ebond‑dist, and Eanch).

energy for the (1 × 4)(9 × 35) cell as Eadh ≅ −1.0 eV (for details see the SI, Figure S7). In a more accurate description, structural irregularity of the real junction and the presence of some excessive (spare) TPA admolecules, not directly engaged in the formation of the interface, have to be considered. Bearing in mind the structure of the dense TPA monolayer on the (110) rutile plane (16 TPA molecules per unit cell), and geometric compatibility between the Zn-MOF-5 (100) and the (110) TiO2 terminations, only one per four TPA molecules shall be involved in the formation of the junction (vide supra). The remaining spare TPA admolecules are separated from the surrounding MOF building units by more than 2.5 Å (Figure S7, SI). To assess their role in the interface energetics, we calculated the interaction of a MOF linker with the adjacent spare TPA admolecules. The resultant energy, mainly due to dispersion forces, was equal to −0.14 eV per unit cell. As a result, in a fair approximation, the spare TPA admolecules can be treated as spectator species interacting nonspecifically with the surrounding MOF layer via dispersion forces, and enhancing the overall energetics of the interface by ca. 6% (see Table 2). Electronic DOS Structure. In a standard account, photoactivity of the bare and doped TiO2, and its more advanced hybrid organic/oxide assemblies, depends critically on the electronic structure, band alignment, and repartition of the density of states. These properties, in particular, provide a background for central discrimination between straddling (Type-I) and staggered (Type-II) interfaces, governing the charge separation and charge transfer events. A comparative analysis of the photoinduced processes on MOF and TiO2 nanoparticles has been reported recently, focusing particularly on oxidation of organic compounds.86 Other examples related to photoactivity can be found elsewhere.55 To reveal the influence of the Zn-MOF-5 layer on the electronic structure of titania in the context of its possible photosensibilization, we calculated the total and partial density of states (DOS) for (110) TiO2, Zn-MOF-5, and the Zn-MOF5(100)/(110)TiO2 interface (Figure 4). The calculated band gap for the (110) TiO2 surface was equal to 1.1 eV (Figure 4, bottom panel). As is well established,70 the maximum of the valence band (VBM), emphasized in this figure by red shadowing for the clarity, is dominated by the 2p orbitals of the surface oxygen anions slightly hybridized with the Ti 3d orbitals, whereas the minimum of the conduction band (CBM) is formed by empty 3d states of the surface Ti cations (green shadow), with appreciable contribution of the oxygen 2p states at higher energies only. The calculated band gap of the bulk Zn-MOF-5 (Figure 4, upper panel) was found to be ∼3.2 eV, close to the experimental value of 3.4−3.5 eV.87,83 The distribution of various electronic states within the valence and conduction bands revealed that binding combinations of the carbon and

Figure 4. Plots of the atom-projected and total density of states for Zn-MOF-5 (upper panel), Zn-MOF-5(100)/(110)TiO2 interface (middle panel), and (110) termination of rutile (lower panel). Color coded lines stand for atom-projected DOS, whereas gray shapes indicate the total DOS.

oxygen p states together with small admixture of the Zn d states contribute to the top of the valence band (orange and blue shadow), whereas the bottom of the conduction band is essentially composed of antibonding combinations of the carbon and oxygen 2p states of the carboxylic groups (orange and blue). It is separated from the next band located at 4−5.1 eV, and constituted by the oxygen and carbon 2p states of the aromatic rings. The DOS diagram of the Zn-MOF-5(100)/(110)TiO2 interface is presented in the middle panel of Figure 4. The CBM states at ∼1.5 eV are again utterly built up of the Ti d orbitals. Formation of the Zn-MOF-5 layer on the rutile surface introduces, however, new electronic states at VBM, lowering the band gap to 0.9 eV. Indeed, the states from −0.5 to 0.0 eV are mainly composed of Cphenyl 2p contributions, associated with the linkers (orange shadow) with a small admixture of O(MOF) 2p (navy blue line), indicating a photonic sensitization of the TiO2 substrate by the MOF layer. The contribution of F



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Figure 5. Electron density contours for the VB edge (blue) and CB edge (yellow) states. Overall picture (a), magnified bonding π-type orbitals of the aromatic BDC linkers; (b) and dxy-type orbitals of the Ti atoms (c).

Figure 6. Simulated HR-TEM images of the MOF-5(100)/TiO2(110) interface along [001] (a) and [11̅0] directions (b), together with respective projected structural models (a2, b2). The optimal simulations settings are as follows: for a1 Cs = −0.012 mm, Δf = 2.4 nm, for b1 Cs = +20 μm, Δf = +12 nm.

Figure 5, as an illustration. Such structure of the energy levels together with a rather strong electronic coupling between the Zn-MOF-5 layer and the TiO2 substrate via covalent bidentate C−O−Ti bonds are favorable for electron injection from the photoexcited Zn-MOF-5 into the sensitized titanium oxide, and for interfacial charge delocalization as discussed by Cao et al.88 The overall charge transfer from MOF to TiO2 was calculated on the basis of Bader population analysis. We calculated the overall charge on the atoms constituting the rutile substrate, and normalize this value to the surface area of the interface, which gave the charge density of 2.04 |e|/nm2. As a result the Zn-MOF-5(100)/(110)TiO2 interface can be classified as an electronically coupled Type II injection system,89 where electron injection to the titania conduction band occurs via interfacial charge transfer, directly from the highest occupied

the Zn d states to DOS structure is mainly confined to the region of 5 eV below the Fermi level. As a result, the top of the valence band of the MOF-5(100)/ (110)TiO2 assembly is shifted up by 0.2 eV, whereas the conduction band minimum remains in essence intact, yet being situated well below the CBM (3.2 eV) of the parent Zn-MOF5. Thus, the optical absorption of the MOF-5(100)/(110)TiO2 hybrid moves to longer wavelengths. Furthermore, there are no localized states located in the band gap that can trap charge carriers influencing the optoelectronic response of this material. The electron density contours for the states located at the very bottom of the conduction band (dxz orbitals of the t2g manifold) and the very top of the valence band (bonding type π orbitals of the aromatic BDC linkers) which can be involved in the photocharge transfer processes are shown in G



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The DOS structure reveals that VBM is composed of carbon 2p states of the BCD linkers, whereas CBM by titanium 3d (t2g) states. The resultant Zn-MOF-5(100)/(110)TiO2 assembly exhibits a staggered (Type-II) system, where the photoinduced electron may be directly transferred from the Zn-MOF-5 part to the conduction band of the sensitized TiO2.

MOF carbon states to the empty states constituted by the Ti 3d orbitals (Figure 5). The higher CBM (LUMO) of Zn-MOF-5 as compared to TiO2 is beneficial for efficient electron injection.90 In addition, a versatile and tunable MOF structure opens a wide route for facile on purpose tailoring of the resultant photonic properties, making the MOF/oxide platform a promising candidate for further improvement of DSSC and other photodevices. The moisture sensitivity of Zn-MOF-5 should not pose forbidding problems for its application in the DSSC devices. The photoactive part is in fact sandwiched between the electrodes that together with the antireflection coating provide the necessary shielding against harmful influence of the environment (dust and humidity).91 We expect that particular improvements could be made by profiting from enhanced ability to hierarchical integration of the structure and functionalities of the MOF/TiO2 assemblies, their lower tendency to aggregate, and better thermal stability, compared with those of conventional dye-sensitized TiO2 structures. Simulation of HR-TEM Pictures of MOF-5 Thin Films on TiO2. Since the Zn-MOF-5(100)/TiO2(110) interface is composed of heavy (Ti, Zn) and light (O, C) atoms, we focused our initial attention on outlining preliminary setting conditions for optimal TEM imaging with the phase contrast originating from the interfacial atoms. Analysis of the defocus vs. thickness maps of the simulated images (Figure S8, SI) revealed that the [110] projection is rather uninformative for observation of the interface structure, whereas in the case of the [100] and [11̅0] directions a great many structural details may be captured at the defocus values ranging from −4.8 to 4.6 nm, and a sample thickness between 10 and 25 nm (Figure S9, SI). Under such imaging conditions one can, in principal, simultaneously observe the Ti, Zn, and oxygen atom columns, and what is most important, the organic BDC linkers can be detected as well. For the optimal sample thickness of ∼13 nm, in the [001] and [11̅0] projections (see Figure 6, panels a1 and b1) the MOF-5 oxygen ions, attached directly to the TiO2 surface, are arranged parallel to the imaging direction, and could be well revealed in the HR-TEM picture. It is also possible to visualize the BDC linkers (light gray contrast), which supports the previous observation that HR-TEM is sensitive to covalent bonding effects.75 The imaging conditions proposed in the present paper are close to those used by Wiktor et al.23 for HR-TEM cryoimaging of the bulk MOF-5 structure, yet the latter gave slightly worse results when applied for the present simulation of the MOF-5(100)/TiO2(110) interface.

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ASSOCIATED CONTENT

* Supporting Information S

Zn-MOF-5 and TiO2 rutile bulk structures descriptions, size and bonding compatibility of TiO2(110) and Zn-MOF-5(100) surfaces, and simulation details for HR-TEM images. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +48 12 663 20 73. Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was carried out within the CMST COST Action: CM1104 − Reducible Oxide Chemistry, Structure and Functions, and supported by Iuventus Plus project of MNiSzW, Grant No. 0633/IP3/2011/71 (simulation of HR-TEM images).

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REFERENCES

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CONCLUSIONS The geometry, electronic structure, chemical bonding, and energetics of the Zn-MOF-5(100)/(110)TiO2 interface were successfully modeled by using periodic PW91/DFT+D calculations. Correct adhesion energy of the Zn-MOF-5 layer to rutile substrate (equal to −0.32 eV/nm2) can be recovered only if the dispersion forces (−0.39 eV/nm2) are fully taken into account, to counterbalance a considerable stress (0.69 eV/ nm2) induced by the interfacial strain (ε[001] = 0.31% and ε[11̅0] = 2.86%). To produce a coherent junction, the supple ZnMOF-5 moiety (with the periodicity of 12.94 Å) is adjusted along the [11̅0] direction to the pattern produced of uneven spacing defined by five (14.80 Å) and four (11.84 Å) surface Ti5c ions, by tilting (10°) and rotating (34°) the BCD linkers. H



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